1. Field of the Invention
The present invention relates to multi-piece fasteners and more particularly to a high strength blind bolt of the threaded torque type or the pull type using a swaged construction and providing a high final clamp load of workpieces secured together. Such blind fasteners, while of general utility, can be particularly useful in constructions utilizing closed sections such as box beams or columns, bridges and the like and more particularly in applications where accessibility on one side of the workpiece is blocked or limited.
2. Description of the Related Art
The fastener of the present invention also relates generally to multi-piece blind bolt fasteners which include a main sleeve, an expandable sleeve and a pin wherein the expandable sleeve is expanded radially to form a blind head in response to a relative axial force applied via the pin. In this regard, the blind bolt of the present invention is related to U.S. Pat. Nos. 5,603,592, issued Feb. 18, 1997 for High Strength Blind Bolt With Uniform High Clamp Range Over An Extended Grip Range ("the '592 patent"); 5,178,502, issued Jan. 12, 1993 for High Strength Blind Bolt ("the '502 patent"); and 5,213,460, issued May 25, 1993 for High Strength Blind Bolt With Uniform High Clamp Over An Extended Grip Range ("the '460 patent"), each of which is incorporated by reference herein. In U.S. Pat. No. 5,603,592, a blind bolt construction is disclosed utilizing a tension controlled type threaded nut and core pin construction. A nut member is threadably engaged on a threaded portion of the shank of the pin. A splined portion is located at the terminal end of the pin shank. An installation tool having a central socket member adapted to engage the splined portion and an outer wrench member adapted to grip the nut is used to apply torque between the core bolt or pin and the nut to provide relative rotation by which the nut is threaded onto the pin to attain the desired clamp up. A pair of sleeves, an expandable sleeve and a main sleeve, located on the pin cooperate to form a blind head in response to the applied torque.
As a result of the applied torque between the nut and the pin, the nut initially transmits an axial force to the main sleeve via a grip adjuster. At the same time, the pin has an enlarged head which transmits an opposite axial force against the expandable sleeve. As the torque and resultant relative axial force increases, the expandable sleeve, reacting against the main sleeve, is radially expanded to form a blind head. A blind head of uniform shape is provided over the extended grip range of the fastener.
At this stage of the installation, the workpieces have not been subjected to any clamp loads between the nut and grip adjuster and the blind head via the pin. In order to bring the blind head into engagement with the blind side surface, the grip adjuster is provided with a frangible, resistance or shear member which is in blocking communication with a cavity. The frangible shear or resistance structure is adapted to fracture at a preselected magnitude of relative axial force whereby the cavity which is no longer blocked is now accessible permitting axial movement of the main sleeve away from or out of load bearing relationship with the expandable sleeve; this permits the fully formed blind head to be brought into contact with the blind side surface of the workpieces with no axial restraint from the main sleeve which has moved substantially freely out of load bearing engagement with the expandable sleeve. Now the workpieces are pulled together by the further application of torque with the resultant relative axial force acting substantially solely between the blind head through the pin and the nut. The force clamping the workpieces together continues to increase until a magnitude of torque is attained at which a splined, pintail portion on the core bolt or pin is fractured from the pin. This fracture load determines the final clamp load on the workpieces.
This structure provides a wide grip range capability, in which a high strength blind head of a uniform structure is formed over this wide grip range. The blind head of uniform structure has the same blind side protrusion over the grip range. In addition, the blind bolt construction provides a structure in which high strength materials can be used resulting in a high strength fastener while at the same time achieving a high, uniform clamp force.
However, the fastener requires two sleeves to operate. The expandable bulb sleeve is made of a relatively soft steel capable of bulbing at low loads and also capable of forming around the head of the pin. The main sleeve is made of a hardened steel capable of acting as a punch to shear the shear member.
The '592 patent discloses that a one-piece sleeve may be used (see col. 10, lines 40-42) but does not describe such an embodiment. Similarly, the '460 patent discloses that a one-piece sleeve may be used (see col. 16, lines 13-15) but does not describe such an embodiment. Thus, given the different requirements for the two sleeves (the main sleeve must be hardened to shear the shear member, the expandable sleeve must be soft enough to bulb and form a blind head), both patents disclose manufacturing the main sleeve and the expandable sleeve from different steels, each having a different hardness. Neither the '592 nor the '460 patent discloses how to make a one-piece sleeve from a single type of steel meeting all of the requirements of the two types of sleeves it would replace.